The invention relates to gamma-ray detection, and in particular to gamma-ray detection for determining the direction to a source of gamma-rays.
There are a number of situations in which there is a desire to be able to determine the characteristics of gamma-ray radiation in an environment. Such characteristics might include the intensity of radiation in the environment, the nature of the radioisotope(s) producing the radiation, and the direction from which the radiation is coming. For example, this information can be of great benefit to emergency staff entering a ‘disorganised’ nuclear environment, for screening personnel and cargo in order to police the illicit trafficking of radioactive materials, and for general searching for ‘orphaned’ sources of radiation.
Gamma-ray detectors with the ability to measure the intensity of radiation in an environment and to identify the nature of the source emitting the radiation (from spectroscopic information) are generally available, for example the GR-135 Exploranium® instrument from the SAIC Corporation, the Indentifinder-2 from Flir Systems Inc., and the Radseeker® instrument from Smiths Detection Inc. However, to determine the direction to the source using these detectors the user must rely on dose-rate trends. This is done by moving the detector and noting how the measured intensity changes. An increase in measured intensity indicates the motion is towards the source. A decrease in measured radiation indicates the motion is away from the source. The location of the source may thus be found by trial and error as a user wanders about in the environment. A problem with this approach is than it can be slow and unreliable.
There have also been proposed gamma-ray detectors with direction finding capabilities that are not reliant on dose trending.
One example gamma-ray detector with direction finding capability is described by Larsen et al [1]. In this device the functions of gamma-ray spectrometry for determining the nature of radioactive isotopes and gamma-ray direction finding are performed independently. A small LaBr3 crystal spectrometer is used to identify the nature of the source whilst the direction towards the source is determined using four Geiger counters. These are located in the four corners of a lead collimator that takes the form of a cross. The count-rates from these four Geiger counters can be used to provide information on the direction towards the source.
Another example gamma-ray detector with direction finding capability is described in WO 2008/015382 [2]. This example uses four scintillation crystals closely-packed around a pointing axis for the detector. When detector signals from the four scintillation crystals are equal the pointing axis of the device is assumed to be aligned with a direction to the source of radiation.
Another example gamma-ray detector with direction finding capability is described by Wahl and He [3]. With this example detector directional information on the source of gamma radiation can be provided using information provided by recording multiple Compton-scatter events in a material such as CdZnTe single crystals. The detector provides information on the energy and location of energy deposits produced by Compton electrons. The kinematics of Compton interactions means a single gamma-ray event can produce an annular ring response. By observing multiple events, a series of intersecting rings may be generated. A most-probable direction to the source of the events may then be derived based on where there is the highest number of intersections among the rings.
Whilst the known approaches for determining directions to sources of gamma-ray radiation can provide useful results, there are some drawbacks associated with the various approaches, for example in terms of device size and complexity.
Accordingly, there remains a need for new designs for radiation detectors that allow the direction from which radiation is coming to be determined, for example using simple hand-held instruments or potentially larger, vehicle mounted detector systems.